The battery packs for portable power tools, outdoor tools and certain kitchen and domestic appliances may include rechargeable batteries, such as lithium, nickel cadmium, nickel metal hydride and lead-acid batteries, so that they can be recharged rather than be replaced. Thereby a substantial cost saving is achieved.
Nevertheless, problems are still encountered by the user. Frequently, the user discovers that the batteries have self-discharged and need recharging at exactly the moment when the user would like to use the device, and recharging in most instances takes an inconveniently long period of time.
One solution to this is to provide maintenance charging systems in which the battery can be left on constant charge between uses. Even this system is of no value if the user fails to put the battery back on charge after use; in addition, most maintenance charging systems actually cause slow deterioration of the battery with time.
The solution to all of the above problems would be the provision of an adequate fast charging system which would reliably bring the battery up to its full state of charge in the shortest possible time and without risk of damage. While the prior art is replete with attempts to provide good fast charging systems, no satisfactory system has yet been developed. Most fast charging systems today require very special conditions, such as unusually expensive batteries which can accept the output of the fast charge system. Even under these special conditions, there remains a risk of serious damage to either the battery or to the charger. In addition, the present fast charge techniques do not properly charge the batteries. Depending on the termination mode used, all fast charge techniques of which we are aware either overcharge or under charge the battery, either of which causes gradual deterioration of the battery and premature failure.
In part, the failures of the prior art have been due to the inability to accurately indicate full battery charge; this has been due either to the failure of the prior art to select the proper mode of indication, or to the fact that, even if a reasonably good indicator has been selected, the charging requirements of a battery vary substantially with individual cell chemistry, with individual cell history and with ambient temperature. Thus, even an indication mode which is reasonably well selected for a particular battery type may actually provide an accurate indication only for a few cells having ideal characteristics and only if the cells are charged under proper conditions of ambient temperature.
For example, a major category of previous fast charging systems has relied upon temperature cutoff to terminate the fast charge mode. However, these systems are subject to several difficulties: they may damage the batteries due to the constant repetition of high temperature conditions, even in specially manufactured (and expensive) cells which are theoretically designed to accept high temperatures; such systems may not be safe for use with defective cells; they actually do not charge a battery to its full capacity, in high ambient temperature conditions; the charge efficiency is low and the systems are therefore wasteful; and in low ambient temperature, the battery may be driven to self-destruct by venting or possibly explosion.
Another major category of prior art fast charging systems relies on voltage cutoff. However, in many types of battery systems including nickel-cadmium, this termination mode is unreliable due to the large voltage variation which can occur with temperature, or due to cell history or individual cell characteristics. Thus, a voltage cutoff system can destroy a battery through venting. Except in unusual ideal conditions, it will never properly charge a battery to its full capacity.
A third major category of prior art battery charging termination is based on simple passage of time. However, the accuracy of this system depends on the battery, at the beginning of charge, having an assumed state of charge. There is a very high likelihood that this will not be the case and that the battery will be either over or under charged.
Most other charging methods which have been used to date are based on combinations of one or more of the above techniques. While some problems can be avoided by these combinations, at least some of them still exist. Even the best fast charge systems require expensive cell constructions; but the additional cost only serves to delay the battery deterioration which is caused by the charging system.
A more recent technique, illustrated by U.S. Pat. No. 4,052,656, seeks the point at which the slope of the voltage-versus-time curve for a given battery is zero. However, even this technique is subject to difficulties; it may detect another point at which the voltage slope is zero but at which the battery is only partially charged; in addition, even if it properly locates the zero slope point which is close to full charge, this inherently overcharges the battery and will cause battery deterioration due to heating.
Typically, most battery charging systems embody one or another of the above techniques and are subject to one or more of the above-listed defects. This is true despite the fact that most currently known battery chargers are designed to be used with only one type of battery and, in general, with only one selected number of battery cells of that particular type. The concept of a battery charger which can accurately and rapidly deliver full charge to a variety of different batteries including different number of cells or different types of battery couples is totally beyond the present state of the battery charging art.
U.S. Pat. Nos. 4,392,101 and 4,388,582 (hereinafter “the Saar Patents”) disclosed a solution to these problems. In particular, the Saar Patents disclosed a new method based on the inflection points of the charging curve.
In the course of recharging a nickel-cadmium battery, it has been found that a very typical curve is produced if the increasing battery voltage is plotted as function of time. FIG. 1 is a representation of a typical curve of this type, as taken during a constant current charging cycle. A similarly typical curve can be obtained by plotting current against time during a constant voltage charging cycle, and a reproducible pattern also occurs if neither voltage nor current are held constant This curve may be divided into significant regions, as indicated by the Roman numerals between the vertical lines superimposed on the curve. While the curve is subject to variations in specific values of voltage or of time, the general form is similar for all nickel-cadmium batteries including one or more cells, and the following discussion applies equally to all such batteries.
Region I of FIG. 1 represents the initial stage of voltage change which occurs when the charging cycle is first started. In this Region, the voltage is subject to significant variations based on the initial charge level of the battery, its history of charge or discharge, etc. Since the shape of this Region can vary, it is indicated in FIG. 1 by a dotted line.
Because the information in Region I varies, it may be preferable to ignore this segment of the curve. The battery will generally traverse Region I completely within the first 30 to 60 seconds of charging and enter Region II; in general, the voltage in the Region I and period increases relatively rapidly from the initial shelf voltage and the short peaks which may occur in this Region are not harmful.
As the battery approaches a more stable charging regime, it enters the portion of the curve designated Region II. Region II may be of fairly long duration with little or no increase in voltage. During this time, most of the internal chemical conversion of the charging process takes place. When significant portions of the active material have been converted, the battery begins to approach full charge and the voltage begins to increase more rapidly. The inflection point A in the curve from a decreasing rate of voltage increase to an increasing rate of voltage increase is identified as the transition from Region II to Region III.
Region III is characterized by a relatively rapid voltage increase as more and more of the active material is converted to the charged state. As the battery approaches full charge more closely, that is, when perhaps 90 to 95% of its active material has been converted chemically, oxygen begins to evolve. This produces an increase in the internal pressure and also an increase in the temperature of the cell. Due to these effects, the rapid increase in battery voltage begins to slow and another inflection point occurs in the curve. This second inflection point is identified as the transition point between Regions III and IV, point B.
Within Region IV, the final portions of the active material are being converted to the chemical composition of the fully charged battery. At the same time, due to oxygen evolution from material already converted, the internal pressure increase and the heating contribute to a slowing in the rate of voltage increase until the voltage stabilizes at some peak value for a short period of time. This is designated as the transition between Regions IV and V.
Within Region V, if charging is continued, the voltage of the cell starts to decrease due to additional heating as virtually all of the applied energy is converted into heat and the negative temperature coefficient of the battery voltage causes the voltage to decrease. Continued application of charging energy in this Region would eventually cause damage to the battery, either through venting or damage to the separator.
As previously noted, the relative time duration, slope or value of any portion of this curve may be modified by such factors as the initial temperature of the battery, the charge or discharge history of the battery, the particular manufacturing characteristics and the individual characteristics of the battery cell. However, the major aspects of this curve and of each of its Regions will be identifiable in any non-defective nickel-cadmium battery which is brought from a substantially discharged state to a fully charged state at a constant, relatively high current.
The charging method disclosed in the Saar Patents basically involves identifying exactly the conditions in the particular battery undergoing charge and correspondingly controlling the application of charge current. As applied specifically to nickel-cadmium batteries, the method of the Saar Patents involves the identification of the inflection point between Regions II and III and by the identification of the subsequent or following inflection point between Regions III and IV. Once these two inflection points have been identified and it has been confirmed that their occurrence is in exactly that order, and only then, the battery charging current can be discontinued or reduced to a maintenance or topping mode if desired, with absolute assurance that the battery has been brought to a full state of charge regardless of its temperature, history, or individual cell characteristics. Because of the accuracy of this determination, this method can even be applied to batteries which are constructed for use only with trickle chargers.
The Saar Patents also disclosed identifying the changes of sign of the second derivative of the voltage with respect to time. Specifically, Region II is characterized by the gradual decrease of the slope or rate of charge of voltage versus time. For a fully discharged battery, Region II constitutes the largest portion of the charging period with voltage over most of this period increasing at a relatively low rate. As the battery approaches full charge, the voltage again starts to increase somewhat more rapidly. Thus, the slope which had been becoming progressively smaller and smaller starts to become larger again. This can be described as an inflection point or a change in sign of the second derivative of voltage with respect to time. Thus, we have a first such change in sign giving indication that the battery is nearing the full charge state.
During Region III the slope of the voltage-time curve increases further and further as the battery comes closer to full charge. At or near the full charge point, there is the transition between Regions III and IV at which the slope of voltage stops increasing and starts decreasing to smaller and smaller values as Region IV progresses. Here again, a change in the sign of the second derivative of the voltage-time curve occurs. This decreasing slope in Region IV indicates that virtually all of the active material in the cell has been changed to the charged state and that the energy going into the cell is beginning to convert into heat rather than continuing the charging process. Thus it is desirable to terminate charge during the early or middle part of Region IV of the voltage time curve.
These two above described changes in sign of the second derivative of the voltage-time curve are characteristic of nickel-cadmium and other electrochemical cells during the charging process. They provide a unique and reliable indication of the state of charge of the battery. A particularly important aspect of the method of the Saar Patents is, accordingly, the use of these observable changes of sign of the second derivative of the voltage-time curve to determine when to terminate battery charging.
However, the Saar Patents are not a panacea. Indeed, the Saar Patents note that: “In some cases, the inflection point technique which is appropriate for normal conditions may not be adequate, for example, if a battery is damaged or defective or if a user inadvertently places a fully charged battery on charge.”
One possible situation is when a “sleeper” cell in a battery pack “wakes up” sometime after the charging process has started. When this occurs, the battery charger will note a sudden increase in the battery voltage, otherwise known as a “voltage step” C, as shown in FIG. 2.
In such circumstance, the charger may interpret the beginning of voltage step C, i.e., point CP1, as the first inflection point, i.e., the point where a decreasing rate of voltage increase changes to an increasing rate of voltage increase. Furthermore, the charger may interpret the end of voltage step C, i.e., point CP2, as the second inflection point, i.e., the point where an increasing rate of voltage increase changes to a decreasing rate of voltage increase. Under the methodology of the Saar Patents, since the charger found the two inflection points, it would be enabled to stop charging soon thereafter, even though the battery pack has not been fully charged.